30. Summary of Hipparcos results

We have used the Hipparcos parallaxes of nearby Galactic Cepheids to
explore corrections to the multiwavelength Period-Luminosity relations
for LMC Cepheids. The latter are based on an LMC data set scaled to a
true distance modulus of 18.50 mag and an adopted foreground reddening
of E (B-V) = 0.10 mag. Although the current uncertainties in the
parallaxes are large and still dependent upon the specific subsets of
the Cepheids chosen for the comparison, the agreement is good,
indicating that to within ± 0.14 mag (or, 7% in distance) the
previously adopted zero point is substantially correct. Based on
different subsamples of data either having BV, BVJK,
BVIJK or BVIJHK photometry, LMC moduli, ranging from 18.44
to 18.57 mag are derived. These results, summarized in
Table 2, differ
from the value of 18.70 mag of
FC97,
which are based solely on the
reddening-corrected V photometry of
Caldwell & Laney (1991),
externally adjusted for metallicity. The Hipparcos data alone do not
allow us to discriminate between metallicity effects and the
physically distinct possibility of added reddening to the LMC.

To alleviate the ambiguity posed by the need to simultaneously solve
both for reddening and metallicity effects on the Cepheid distances we
are currently deriving OB-star reddenings along the individual lines
of sight to several dozen LMC Cepheids. This will allow us to
decouple the reddening determinations from metallicity effects, and go
beyond the use of a single mean (foreground + internal) reddening
for the LMC calibrating Cepheid sample. Preliminary
reductions
indicate that the variance from field to field is large (ranging from
E (B-V) = 0.00 up to 0.40 mag) while still indicating that an average
value of < E (B-V) > = 0.10 mag is appropriate for
the LMC calibrating Cepheids. Details will be presented
in Madore, Freedman
& Pevunova (1998 in preparation).

We close by noting that at least three other very recent
determinations of the true modulus to the LMC fall on either side of
the value 18.50 mag adopted by
MF91
in setting a zero point for the
Cepheid distance scale. Both
Reid (1997) and
Gratton et al. (1997)
derive large LMC moduli (18.65 ± 0.10, and 18.63
± 0.06 mag, respectively) using Hipparcos-based calibrations of
the Galactic globular cluster and RR Lyrae distance scale. On the
other hand,
Gould & Uza (1997)
have re-analyzed the SN 1987A
supernova ``light echo'' and derive an upper limit of
µ LMC < 18.37 ± 0.04 mag for the LMC true
distance modulus; although they note that if the ring is slightly
elliptical (b / a ~ 0.95)
this upper limit increases to < 18.44 ± 0.05 mag. A value of
18.56 ± 0.05 mag has been derived by
Panagia et al. (1996)
from the same data. Until these differences are fully understood and
resolved, and given the remaining uncertainties in the Hipparcos
Cepheid parallax data, we prefer to adopt a true distance modulus of
18.50 mag for the LMC, but now bounded by an uncertainty of
± 0.15 mag, defined to fully encompass the above range of recently
published values. This value is consistent with other estimated
distances to the LMC based on a wide variety of methods (for a
comprehensive modern review see
Westerlund 1997).
Viewed in that
perspective, the Hipparcos data confirm the Cepheid distance scale at
better than the ± 10% level (95% confidence).